Along with development of theories such as radiophysics, radiobiology, clinical oncology, etc., especially with continuous development of medical imaging apparatus and computer technology, radiotherapy (RT) technology has been continuously developed, and meets more clinic requirements. It is a great improvement from the conventional RT technology to 3D conformal radiation therapy (3DCRT). Since then, RT technology has entered into an era of precise RT, and incidence rate of partial recurrence of tumor and complication of normal tissue has been largely reduced. Intensity-modulated radiation therapy (IMRT) developed based on 3DCRT can conform to target better and protect organs at risk (OAR) around the target better, especially when the OAR is positioned in a concave target.
The IMRT, it may include one or more steps: dividing a beam field into a plurality of segment fields, giving these segment fields different weights, and generating an uneven intensity distribution in the beam field. In this way, beam fluence aimed at the OAR may be reduced while beam fluence aimed at other portion of the target may be increased. In a radiotherapy apparatus, a desired dose distribution may be implemented by controlling a multileaf collimator (MLC) according to a fluence map. An MLC may include a plurality of leaf pairs. Each leaf pair may include a trailing leaf and a leading leaf. The trailing leaf may be positioned within a carriage on the left, and the leading leaf may be positioned within a carriage on the right. The trailing leaf and the leading leaf may extend out of the corresponding carriages. The lengths of extension of the trailing leaf and/or the leading leaf may be restrained. Difference between the longest length of extension and the shortest length of extension of the trailing leaf or the leading leaf may be referred to as the largest leaf-span.
In a radiotherapy, if a tumor has large coverage areas and complex shapes (e.g., head-neck tumors), the fluence maps obtained by modulating intensity may usually have large coverage areas and complex shapes. For one of the fluence maps, if the horizontal distance between initial and/or terminal points in rows to be irradiated is greater than the largest leaf span of an MLC, irradiation of the initial and/or terminal points in the rows may not be started and/or completed simultaneously.
At present, a field-dividing method may be used to solve the problems. The method may be implemented by dividing a beam field into a plurality of segment fields. During irradiation, jaws and an MLC moves to a first segment field, and the first segment field may be irradiated firstly. When the irradiation of the segment field is completed, the irradiation is turned off (e.g., the machine that supplies the irradiation is turned off). Then the jaws and the MLC may move to a second segment field, and the second segment field may be irradiated. The above operations may be repeated until irradiation of all segment fields being completed. The jaws may include parallel jaws (the movement direction of the jaws may be parallel to the movement direction of the MLC) and perpendicular jaws (the movement direction of the jaws may be perpendicular to the movement direction of the MLC). The jaws may be used to define a range of a beam field.
However, the field-dividing method may have some defects. In the method, a beam field may be divided into a plurality of segment fields. Irradiation of the plurality of segment fields may increase total Machine Unit (MU). For example, the total MU may be approximately twice the original minimum total MU when one beam field is added. Besides, if irradiation of a segment field is completed, the jaws and the MLC may move to next segment field. The moving of the jaws and the MLC may take time. The time spent on the moving may be referred as a “set-up time.” The total treatment time, which is approximately the summation of the set-up time and the irradiation time, may be increased accordingly. Further, there may be penumbra at edges of the segment fields. Thus, doses delivered at the edge of adjacent segment fields may be inaccurate.